Automotive bodies, home electrical appliances and the like have been manufactured in which metal materials such as steel sheets, galvanized steel sheets, and aluminum-based metal materials are made into a molded metal form, and thereafter painting, assembly and the like are performed. The painting of such a molded metal form is performed through various processes such as degreasing, surface conditioning, chemical conversion treatment, and electrodeposition coating.
The surface conditioning is performed for the subsequent chemical conversion treatment, in which a chemical conversion coating film made of phosphate crystals is formed on the entire surface of the metal material uniformly, quickly and with high density. Generally, in surface conditioning, phosphate nuclei are formed on the surface of a metal material by dipping into a treatment liquid for surface conditioning. As a treatment liquid used for such a surface conditioning treatment, a composition is known in which bivalent or trivalent metal phosphate is combined with various stabilizers (e.g., Patent Document 1, Patent Document 2 and Patent Document 3).
Patent Document 1 discloses a pretreatment liquid for surface conditioning used before the chemical conversion treatment of a metal, which has a pH adjusted to be 4 to 13, and which includes: at least one selected from phosphate including at least one kind of bivalent or trivalent metals including a particle of a diameter of no more than 5 μm; an alkali metal salt, an ammonium salt or a mixture thereof; and at least one selected from the group consisting of an anionicly charged and dispersed oxidant fine particle, anionic water-soluble organic polymer, nonionic water-soluble organic polymer, anionic surfactant, and nonionic surfactant.
Patent Document 2 discloses a treatment liquid for surface conditioning before a chemical conversion treatment, which contains at least one kind of phosphate particle selected from phosphate containing at least one of bivalent and/or trivalent metals, and which further contains at least one kind selected from monosaccharide, polysaccharide and a derivative thereof; orthophosphoric acid and polyphosphoric acid or an organic phosphon acid compound and at least one kind of water-soluble polymer compound consisting of a polymer or a derivative of vinyl acetate, or a copolymer of monomer, which is copolymerizable with vinyl acetate, and vinyl acetate; or a polymer or copolymer resulting from polymerization of: at least one kind selected from a particular monomer or a, β unsaturated carboxylic acid monomer, and no more than 50 mass % of a monomer which is copolymerizable with the monomer. Moreover, Patent Document 3 discloses a surface conditioning composition in which clay mineral is used together with phosphate.
However, even the treatment liquids for surface conditioning disclosed in these documents may not have sufficient chemical conversion properties. For example, in the portion where aluminum-based metal materials come in contact with steel sheets or galvanized steel sheets, the aluminum-based metal materials become an anode, and the steel sheets or galvanized steel sheets become a cathode, and therefore electrochemical corrosion reactions (electrolytic corrosion) tend to occur due to the potential difference of the different kinds of metal. This leads to a problem in that it is difficult to form a conversion coating film on the surface of the aluminum-based metal materials. Due to this, a surface conditioning composition, which can suppress electrolytic corrosion of the metal materials in a conversion treatment, is intended to be developed.
In addition, when these treatment liquids for surface conditioning are applied to conversion resistant metal materials such as aluminum-based metal materials and high-tensile steel sheets, there is a problem in that a sufficient amount of conversion coating film is not formed on the surface of the metal materials. In addition, the required level of corrosion resistance has been increased in recent years, and the formation of a more dense conversion coating film has been desired. Moreover, regarding these treatment liquids for surface conditioning, the particle size of the phosphate particles is large, and the specific surface area is small, and there has been a problem in that phosphate particles in the conversion treatment bath tend to sediment. Due to this, a surface conditioning composition which solves these problems and which has further superior properties has been desired.                Patent Document 1: Japanese Unexamined Patent Application, Publication No. H 10-245685        Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2000-96256        
Patent Document 3: Japanese Unexamined Patent Application, Publication No. S 59-226181